Any ophthalmic lens intended to be held in a frame involves a prescription. The ophthalmic prescription can include a positive or negative power prescription as well as an astigmatism prescription. These prescriptions correspond to corrections enabling the wearer of the lenses to correct defects of his vision. A lens is fitted in the frame in accordance with the prescription and with the position of the wearer's eyes relative to the frame.
In the simplest cases, the prescription is nothing more than a positive or negative power prescription. The lens is said to be unifocal and has a rotational symmetry. It is fitted in a simple manner in the frame so that the principal viewing direction of the wearer coincides with the axis of symmetry of the lens.
For presbyopic wearers, the value of the power correction is different for far vision and near vision, due to the difficulties of accommodation in near vision. The prescription thus comprises a far vision power value and an addition (or power progression) representing the power increment between far vision and near vision; this comes down to a far vision power prescription and a near vision power prescription. Lenses suitable for presbyopic wearers are progressive multifocal lenses; these lenses are described for example in FR-A-2 699 294, U.S. Pat. No. 5,270,745 or U.S. Pat. No. 5,272,495, FR-A-2 683 642, FR-A-2 699 294 or also FR-A-2 704 327. Progressive multifocal ophthalmic lenses include a far vision zone, a near vision zone and an intermediate-vision zone, a principal progression meridian crossing these three zones. They are generally determined by optimization, based on a certain number of constraints imposed on the different characteristics of the lens. These lenses are all-purpose lenses in that they are adapted to the different needs of the wearer at the time. Families of progressive multifocal lenses are defined, each lens of a family being characterized by an addition which corresponds to the power variation between the far vision zone and the near vision zone. More precisely, the addition, referenced A, corresponds to the power variation on the meridian between a point FV of the far vision zone and a point NV of the near vision zone, which are respectively called far vision reference point and near vision reference point, and which represent the points of intersection of viewing with the surface of the lens for far distance vision and for reading vision.
Independently of the power and power addition prescription, a wearer may be prescribed an astigmatism correction. Such an astigmatism prescription is produced by the ophthalmologist in the form of a pair formed by an axis value (in degrees) and an amplitude value (in diopters). On a surface, the amplitude value represents the difference between the principal curvatures; the axis value represents the orientation, relative to a reference axis and in a conventional direction of rotation, of one of the two curvatures according to the formula that is chosen to be used. In practice there are two conventions, the so-called “negative cylinder” convention, in this case, if 1/R1 is the maximum curvature and 1/R2 the minimum curvature, the amplitude value is (1/R2−1/R1) and the axis is the orientation, relative to the reference axis, of the maximum curvature 1/R1, and the so-called “positive cylinder” convention, in this case the amplitude value is (1/R1−1/R2) and the axis is the orientation, relative to the reference axis, of the minimum curvature 1/R2. The reference axis is horizontal and the direction of rotation is the counterclockwise direction when looking at the wearer. An axis value of +45° therefore represents an obliquely orientated axis, which, when looking at the wearer, extends from the top-right quadrant to the bottom-left quadrant. In astigmatism prescription terms, the amplitude value represents the difference between the minimum and maximum powers in a given direction and the axis represents the orientation of the maximum power if the astigmatism value is negative (the axis represents the orientation of the minimum power if the astigmatism value is positive). Such an astigmatism prescription is measured in far vision of the wearer. Although it is linguistically incorrect, the term astigmatism is often used for the amplitude of the astigmatism whereas this term refers to the amplitude/angle pair. The context allows a person skilled in the art to understand which meaning is intended.
Moreover, the laws of the optics of ray tracings mean that optical defects appear when the light rays deviate from the central axis of any lens. These known defects, which include amongst others a curvature or power defect and an astigmatism defect, can be generically called obliquity defects of rays.
A person skilled in the art knows how to compensate for these defects. For example, EP-A-0 990 939 proposes a method for determination by optimization of an ophthalmic lens for a wearer having an astigmatism prescription. This document proposes choosing a target lens and using a ray tracing method and minimizing the difference between the residual astigmatism and the astigmatism of the target lens. Residual astigmatism is defined in this document as the differences in amplitude and axis between the prescribed astigmatism and the astigmatism generated by the lens. This method allows a better adaptation of the lenses to astigmatic wearers, avoiding the optical aberrations caused by the addition of a toric surface. The calculation is carried out at a reference point linked to the eye, which allows account to be taken of the torsion effect of the eye when the wearer looks in an off-centered direction.
The obliquity defects have also been identified for progressive multifocal lenses. For example, WO-A-98 12590 describes a method for determination by optimization of a set of multifocal ophthalmic lenses. This document proposes defining the set of lenses in consideration of the optical characteristics of the lenses and in particular the wearer power and oblique astigmatism, under wearing conditions. The lens is optimized by ray tracing, using an ergorama linking a target object point with each direction of viewing under wearing conditions. This ergorama provides targets for an optimization of the lenses by ray tracing in order to calculate the wearer power and the resulting astigmatism at each point of the lens through which the line of vision passes.